1. Field of Invention
The present invention pertains to a process control system wherein the control engine is a soft PLC (Programmable Logic Controller), in particular a soft PLC written in a programming language based on the IEC 61131-3 standard.
2. Description of the Prior Art
Process control systems wherein a control logic, based on a soft PLC (Programmable Logic Controller) in which functions are defined by software, is used as the control application execution engine are often written in a programming language conforming to the IEC 61131-3 standard, in response to the demand for open systems. A soft PLC is a software model.
FIG. 1 is a function block diagram of a prior art example of a process control system in which a soft PLC is used as the control engine. A control unit 1 comprises an operation means 1a and data input means 1b for the operation means 1a. 
An input output unit 2 receives an input signal (analog signal) related to physical unit values and the like from a process, then transmits the input value to the data input means 1b of the control unit 1 after A/D converting and normalizing it. An I/O bus 3 joins the control unit 1 and input output unit 2.
An operation and monitoring unit 4 has a human-machine interface function, and is joined with the control unit 1 by a control bus 5. The operation value of the operation means 1a is transmitted to a display means 4a, then converted to a physical unit value scale and displayed.
In the control unit 1 and operation and monitoring unit 4, the input value and operation value are provided as data in which the physical unit value span is normalized (0–100%). Therefore, operations and displaying require engineering unit value scale data for the physical unit value.
An engineering unit value scale data setting means 6 sets the engineering unit value scale data for the operation means 1a. An engineering unit value scale data setting means 7 sets the engineering unit value scale data for the display means 4a of the operation and monitoring unit.
Accordingly, the input of a conventional soft PLC is normalized data, so when engineering unit values are to be handled, engineering unit value scale data must be provided to both the operation means and display means and separately converted to physical unit value data.
In particular, in cases where the soft PLC is written in a programming language based on the IEC 61131-3 standard, parameters such as engineering unit value scale data used for the soft PLC must be defined by using external variables. In addition, it is necessary to set the engineering unit value scale data as external variables separately in the control unit 1 and operation and monitoring unit 4, which have independent hardware configurations utilizing a bus connection.
Therefore, if engineering unit value scale data are to be changed, it is necessary to change the engineering unit value scale data information input to both the operation means 1a and display means 4a. If the locations to be changed are not fully recognized due to a mistake, then there will be change omissions, and there will be a problem in that the operation value and display scale are incorrect.
The changing of both settings by an operator essentially involves a time lag, so in the transient state where the data has been changed for only one of them, there are likewise problems in which the operation value and display scale are incorrect.